A typical large scale industrial drilling apparatus comprises a cutting head mounted to a distal end of a hollow boring bar which is concentric with an internal feed screw. The proximal end of the boring bar incorporates a feed nut which engages on the feed screw so that relative rotation between the boring bar and the feed screw causes the cutting head and boring bar to traverse axially along the feed screw whilst the cutting head is rotated for cutting. A main drive motor, typically including a reduction gearbox, is used to rotate the boring bar and cutting head. So-called hot-tap drilling apparatus is often of this design. Such apparatus is for use in forming holes in walls of pipes whilst contents are still inside and perhaps under pressure. The cutting head is typically in the form of a hole saw arrangement and the boring bar is provided with sealing means so that the pressurised contents of the pipe are neither released nor enter the drive arrangements of the hot-tap drilling apparatus.
A typical operation would involve positioning the boring apparatus over the pipe to be drilled and advancing the cutting head to come into contact with the pipe wall. With the cutting head rotating, driven by the main drive motor, the cutting head is advanced into contact with the pipe wall and fed along the axis of the feed screw at a low feed rate such as 0.1 or 0.2 mm/revolution. In order to achieve such feed rates for the cutting head, the difference in rotational velocity between the boring bar and the feed screw may need to be about 1% or less of the rotational velocity of the cutting head and boring bar. Cutting heads may be operated at rotational velocities from say 2 to 20 rpm, or with even higher rotational speeds up to 40 or 60 rpm, with required feed rates from say 0.1 to 1 mm/revolution. Higher feed rates are typically only required for cutting heads with many teeth and these are typically driven at low rotation velocities.
Prior art industrial boring equipment of this type generally has a feed gear box, driven by the main drive motor which drives the rotation of the boring bar/cutting head. The output from feed gear box is arranged to have a small difference in rotational velocity compared to the input rotational velocity from the main drive motor. This output from the feed gearbox is used to drive the feed screw so that a small difference in rotational velocities between the feed screw and the main cutting head drive may be achieved. Most apparatuses have only one ratio available from the feed gear box, so that only one feed rate per revolution is available, though some apparatuses now on the market have up to four different selectable gear ratios to provide distinct, different feed rates.
Another solution in the prior art involves the use of separate drive motors for the cutting head and for the feed screw. This arrangement requires no gear box to apportion the drive between the cutting head and the feed screw. Instead, the cutting head and the feed screw are driven independently by two individual, separate motors. In the prior art these are hydraulic motors. Such motors may be independently controllable with sufficient accuracy to provide a small difference in rotational velocity between drive shaft and cutting head. Control of the feed rate for the cutting head requires close control of the two drive motors because the feed rate is proportional to the difference between the two motor rotation velocities.
Although this solution has the benefit of mechanical simplicity and provides scope for continuous variability of feed rate, it requires either both constant monitoring and manual adjustment of the different motor speeds, or a sophisticated electro-hydraulic control system, in order to maintain the uniform difference between the two motor speeds to provide a constant feed rate for the boring bar/cutting head. This type of boring equipment is typically for use in hostile outdoors conditions, and so any control system, such as a computer, will need to be made extremely rugged to withstand harsh environmental conditions and/or weather. Furthermore, the electrical components essential to a control system mean that the apparatus may need complex sealing systems in place (for instance to avoid risk of explosions when working with gas, or fuels in pipes to be drilled or to avoid risk of short circuits when working on water pipes).
The accuracy required for controlling the speed difference between two drive motors, for the feed rates required, means that an arbitrary speed variation of +/−0.25% in the cutting head and feed screw motor rotation rates can give rise to variations in feed rate of +/−55% or more.
When the cutting head is being used for cutting through a pipe wall, a slow, preferably variable, feed rate is desirable. However, when moving the cutting head towards the pipe wall to commence cutting, or away from the pipe wall after cutting operations have been completed, or during an emergency shutdown, it is desirable to be able to traverse the cutting head axially at much more rapid feed rates than those required for cutting. In other words, it is desirable to be able to provide controlled rapid axial movement in addition to the slow axial feed rates suitable for cutting operations.
Hence there is a need for boring apparatus, and methods for operation of boring apparatus, which address or overcome some or all of the problems mentioned above. In particular, there is a need for boring apparatus which can be operated with a continuously variable feed rate over a wide range of feed rates, with high consistency of feed rate.